1. Field of the Invention
The present invention relates to a repeating station (repeater), a communication apparatus, and a directivity control method for relaying and amplifying a radio signal in mobile communications.
2. Description of the Related Art
In mobile communication systems using an electric wave such as a cellular phone system, there is a problem in that communications are not available out of radio coverage, e.g., in a tunnel. In order to cope with such situations, a radio repeating station, often called a repeater or a booster, is employed. Repeaters are used not only as a measure to expand radio coverage, but also in order to provide a multi-path propagation environment such that the channel capacity of MIMO (Multiple Input Multiple Output) communications is increased, the MIMO communications providing high frequency efficiency in a non-line-of-sight propagation environment.
The configuration of the repeater is described with reference to FIGS. 1A and 1B.
FIG. 1A shows the configuration of a direct relay method wherein a signal received by a receiving antenna is amplified by a low noise amplifier (LNA) and a high power amplifier (HPA), and the amplified signal is transmitted from a transmitting antenna. According to the direct relay method, the frequency of transmission is the same as that of reception; for this reason, its frequency efficiency is high, and it has an advantage in that the configuration is simple. However, a loop interference may take place by the signal feeding back from the transmitting antenna to the receiving antenna, which is a problem in that the gain of the amplifiers cannot be made great.
FIG. 1B shows the configuration of a heterodyne relay method wherein a first signal frequency is received by the receiving antenna, the signal is converted to an intermediate frequency, amplified, and is then converted to a second frequency for transmission. According to this method, the frequency of transmission and the frequency of reception differ; for this reason, there is no feedback from the transmission antenna to the receiving antenna, and sufficient amplification can be provided. However, since two frequencies are used, the frequency efficiency is degraded.
Next, a method of increasing the channel capacity of MIMO communications in a line-of-sight environment by a repeater is described (for example, Patent Reference 1 refers). The MIMO communications employ spatial multiplexing, wherein different data streams are transmitted from i antennas, and are simultaneously received by j antennas (i and j are positive integers). Accordingly, under a multi-path propagation environment wherein two or more independent propagation paths are available between transmission and reception, the channel capacity is increased. However, in a propagation environment where the transmitting station and the receiving station are in line-of-sight, independent propagation paths are difficult to obtain, and the channel capacity is degraded.
FIG. 2 shows a radio communications system that includes repeaters for artificially arranging a multi-path propagation environment in order to increase the channel capacity in the line-of-sight environment.
The radio communications system includes a first radio station (transmitting station), two or more second radio stations (repeaters), and a third radio station (receiving station).
A signal is transmitted from the transmission antennas of the first radio station (transmitting station), and received by the second radio stations (repeaters). Each of the repeaters stores the received signal in a buffer, and transmits the signal after a delay of a predetermined period. Consequently, the signals stored in the buffers are simultaneously transmitted from the repeaters. The signals transmitted from the repeaters are then received by the third radio station (receiving station). The receiving station restores the signal transmitted by performing signal separation of the received signals. In this manner, a multi-path propagation is obtained in the line-of-sight propagation environment, and the channel capacity is increased. However, according to this method, the repeaters transmit by time division, i.e., at a timing that is different from the transmitting station. Consequently, the amount of signals that can be transmitted is approximately halved in comparison with the case where no time division is used, and a transmission delay is inevitable.
In mobile communications, transmission speed keeps increasing, and frequency efficiency is required. Further, information to be transmitted is of a real time nature, i.e., requires the least possible delay. In other words, repeaters to be applied to future mobile communications are required to provide high frequency efficiency, and the smallest possible delay. From this viewpoint, the direct relay method shown in FIG. 1A is suitable as the relaying method; however, there is the problem of the loop interference between the antennas if great amplification is required.
According to the conventional direct relay method, the directivity of an antenna is set in the direction of a base station, and the directivity of an antenna on the other side is set in the direction of a terminal, such that the directional gain between the antennas is reduced. Further, feedback between the antennas is prevented by providing a sufficient distance between the antennas such that propagation loss increases. However, the position of the terminal, which is often in motion, is unknown; therefore, the antenna directivity of the repeater on the side of the terminal is desired to be omni-directional in the horizontal plane.
In the case wherein the repeaters are used in order to realize a multi-path environment for MIMO communications, it is further desirable that the antenna provided on the base station side of the repeater be omni-directional so that the number of propagation paths between the base station and the repeater is increased. When the channel capacity is to be increased in the MIMO communications under line-of-sight conditions, it is seldom necessary to provide a great amplification factor. Nevertheless, in the case of the repeater using the direct relay method, the omni-directional antennas provided to both transmission and reception cause the transmission signal to be fed back to the receiving antenna, i.e., a loop interference takes place.
[Patent Reference 1] JPA 2003-198442
[Problem(s) to be Solved by the Invention]
As described above, problems in the background technology are as follows.
As for the repeater for mobile communications, the direct relay method is suitable from the viewpoint of the frequency efficiency and the transmission delay. Further, when the repeater is to be applied to the MIMO communications for providing a multi-path propagation environment, the omni-directional antennas for transmission and reception are desirable.
However, when using the omni-directional antennas for transmission and reception with the direct relay method, the loop interference takes place from the transmission antenna to the reception antenna, causing an oscillation.